There is a directional microphone device which is a sound-pressure gradient type and is widely used not only as a built-in microphone, but also as a general directional microphone. While the directivity synthesis method of the sound-pressure gradient type has an advantage that a small directional microphone device can be achieved, the method has a disadvantage that sound pressure sensitivity is decreased when signals are synthesized. In the directivity synthesis method of the sound-pressure gradient type, microphone sensitivity decreases with respect to the thermal noise level of a microphone unit and a microphone amplifier at the time of synthesis of the signals. This results in deterioration of signal to noise ratio (S/N). Particularly, when directivity synthesis of the sound-pressure gradient type is performed on output signals from plural microphone units, influences of the thermal noise cannot be ignored, which imposes low-frequency limit of frequency range having directivity and limitation in miniaturization of microphone array.
FIG. 1 is a block diagram showing a conventional directional microphone device 1.
A directional microphone device 1 includes: a first microphone unit 11; a second microphone unit 12; a signal delay unit 14 which receives an output signal from the second microphone unit 12 and delays the received signal; a signal subtraction unit 15 which subtracts the output signal provided from the signal delay unit 14 from an output signal provided from the first microphone unit 11; and a frequency characteristic modification unit 16 which receives the output signal from the signal subtraction unit 15, modifies the frequency characteristic of the received signal, and provides the resulting signal.
The operation of the conventional directional microphone device 1 structured as above is described.
The structure of the conventional directional microphone device 1 shown in FIG. 1 is a basic structure of a microphone which obtains directivity from two microphone units through sound-pressure gradient type synthesis. In FIG. 1, the first microphone unit 11 and the second microphone unit 12 are arranged with a spacing of distance d in the direction opposite to the front direction in the figure.
Let the sensitivity characteristic of the output signal from the first microphone unit 11 be ms1(ω), the sensitivity characteristic of the output signal from the second microphone unit 12 be ms2(ω), the direction of the sound source S be θ (where front is 0°), and velocity of sound be c, the sensitivity characteristic D (θ, ω) of the output signal from the signal subtraction unit 15 with respect to the sound source S can be expressed by the following Formula 1.
                              D          ⁡                      (                          θ              ,              ω                        )                          =                              ms            ⁢                                                  ⁢            1            ⁢                          (              ω              )                                -                      ms            ⁢                                                  ⁢            2            ⁢                                          (                ω                )                            ·                              ⅇ                                                      -                    j                                    ⁢                                                                          ⁢                  ω                  ⁢                                                            d                      ·                                              cos                        ⁡                                                  (                          θ                          )                                                                                      c                                                              ·                              ⅇ                                                      -                    j                                    ⁢                                                                          ⁢                  ω                  ⁢                                                                          ⁢                  r                                                                                        [                  Formula          ⁢                                          ⁢          1                ]            
Here, exp (−jωτ) indicates that signal is delayed by τ. Formula 1 represents, for example, that the output signal from the second microphone unit 12 provided to the signal subtraction unit 15 with delay τ cancels the signal of the first microphone unit 11 depending on the angle θ of the direction of the sound source S. More particularly, Formula 1 indicates that the directional microphone device 1 has directivity.
On the other hand, let the thermal noise characteristic of the output signal from the first microphone unit 11 be mn1(ω), and the thermal noise characteristic of the output signal from the second microphone unit 12 be mn2(ω), the thermal noise characteristic N(ω) of the output signal from the signal subtraction unit 15 can be expressed by the following Formula 2.N(ω)=mn1(ω)−mn2(ω)·e−jωτ  [Formula 2]
Here, mn1(ω) and mn2(ω) are thermal noise of individual microphone unit; and thus, they are independent of each other. Therefore, the average power spectrum of the thermal noise signal is expressed by the following formula. {N(ω)}2≅ {mn1(ω)}2+ {mn2(ω)}2  [Formula 3]
Thus, where the levels of mn1(ω) and mn2(ω) are equal to each other, the average spectrum of N(ω) is mn1(ω) with an approximate increase of 3 dB, that is, approximately twice of the mn1(ω).
Hereinafter, calculation results of the above formulas where the microphone unit spacing d that is a distance between the first microphone unit 11 and the second microphone unit is 10 mm, are shown.
FIG. 2A to FIG. 2C are diagrams showing sound pressure frequency characteristic of each processing block of the conventional directional microphone device 1. FIG. 2A is a diagram showing sensitivity characteristic in the front direction and thermal noise spectrum in the first microphone unit 11 and the second microphone unit 12. FIG. 2B is a diagram showing sensitivity characteristic in the front direction and thermal noise spectrum in the signal subtraction unit 15. FIG. 2C is a diagram showing sensitivity characteristic in the front direction and thermal noise spectrum in the frequency characteristic modification unit 16.
FIG. 3A to FIG. 3C are diagrams showing directional pattern of each processing block of the conventional directional microphone device 1. FIG. 3A is a directional pattern in the first microphone unit 11 and the second microphone unit 12. FIG. 3B is a directional pattern in the signal subtraction unit 15. FIG. 3C is a directional pattern in the frequency characteristic modification unit 16.
Based on the calculation result of Formula 1, the sensitivity characteristic in the front direction in the output signal of the signal subtraction unit 15 is indicated by solid line in FIG. 2B. More specifically, the sensitivity decreases at lower frequency range of longer wavelength, which makes the gradient 6 dB/oct.
On the other hand, based on the calculation results of Formula 2 and Formula 3 which represent thermal noise from the first microphone unit 11 and the second microphone unit 12, the thermal noise in the output signal of the signal subtraction unit 15 is indicated by dashed line in FIG. 2B. More specifically, the thermal noise increases by 3 dB in the calculation result of the signal subtraction unit 15.
Further, due to the relationship between the arrangement of the first microphone unit 11 and the second microphone unit 12 and wavelength of the sound wave, the output signal from the signal subtraction unit 15 has a sound pressure frequency characteristic which decreases with a gradient of 6 dB/oct at lower frequency range. Thus, the frequency characteristic modification unit 16 amplifies low frequency range by 6 dB/oct gradient so as to flatten the sound pressure sensitivity in the front direction as shown in FIG. 2C.
As a result, the thermal noise level of the directional output signal from the frequency characteristic modification unit 16 increases, for example, by approximately 30 dB at low frequency (100 Hz), for the front sensitivity of the same wavelength (see FIG. 2A and FIG. 2C which show sound pressure frequency characteristics).
Further, as shown in FIG. 3A, the output signals of the first microphone unit 11 and the second microphone unit 12 have non-directional directivity, whereas, as shown in FIG. 3B and FIG. 3C, the output signal of the directional microphone device 1 has a unidirectional pattern (where τ=d/c).
In general, the S/N of a widely used electret condenser microphone (ECM) is approximately ranging from 58 to 60 dB (where reference sound pressure level is 94 dBspl (1 kHz)). It is the level at which the thermal noise of the ECM is slightly greater than background noise and which can be auditorily perceived, in a quiet environment of noise level at 30 dB(A) approximately. However, for example, in the case where two microphone units are used for the small directional microphone device 1 with the spacing d of approximately 10 mm between the microphone units, the thermal noise level, as described earlier, increases by 30 dB (at 100 Hz) due to directivity synthesis. As a result, small sound is buried in the thermal noise, thereby not being perceived (sensitivity is decreased). This causes practical issues.
As seen in FIG. 2C, the increase in the thermal noise level becomes larger at lower frequency in principle. Thus, there is another proposed conventional microphone device which has a non-directional directivity and high sensitivity at low frequency and has a directivity only at high frequency (see patent reference 1).
FIG. 4 is a block diagram showing a structure of a conventional directional microphone device 10.
In FIG. 4, the directional microphone device 10 described in patent reference 1 includes: a first microphone unit 11; a high pass filter 13 which receives an output signal from a second microphone unit 11 and is a unit for passing only high frequencies in the received signal; a signal delay unit 14 which receives the output signal from the second microphone unit 12 and delays the received signal; a signal subtraction unit 15 which subtracts the output signal from the signal delay unit 14 from the output signal from the high pass filter 13; and a frequency characteristic modification unit 16 which receives the output signal from the signal subtraction unit 15 and modifies the frequency characteristic of the received signal. Here, the output signal from the directional microphone device 10 is the output signal from the frequency characteristic modification unit 16.
Hereinafter, the operation of the directional microphone device 10 is described.
The conventional directional microphone device 10 shown in FIG. 4 addresses the problem that is decrease in sensitivity at low frequency in the basic structure of the microphone which obtains directivity from two microphone units through sound-pressure gradient type synthesis. The conventional directional microphone device 10 differs from the conventional directional microphone device 1 shown in FIG. 1 in that the high pass filter 13 is provided in the latter stage of the first microphone unit 11. Other structure is the same as that of the conventional directional microphone device 1. Note that in the following description, a case where the first microphone unit 11 and the second microphone unit 12 have non-directional directivity is described.
For high frequency that is a passband of the high pass filter 13, the directional microphone device 10 in FIG. 4 has a structure identical to that of the conventional directional microphone device 1, thereby providing an output signal having directivity. On the other hand, for low frequency that is a stopband of the high pass filter 13, signal is attenuated by the high pass filter 13. More specifically, as the operation of the directional microphone device 10, among the first microphone unit 11 and the second microphone unit 12, only the output signal from the second microphone unit 12 is provided as an output signal. This results in the directional microphone device 10 that has, for low frequencies, non-directional directivity of the second microphone unit 12, and has, for high frequencies, directivity of primary sound-pressure gradient type obtained through synthesis of the signals from the first microphone unit 11 and the second microphone unit 12.
FIG. 5 is a diagram showing sound pressure frequency characteristic of the conventional directional microphone device 10.
In FIG. 5, the sound pressure sensitivity characteristic in the axial direction of the directivity and the spectrum of the thermal noise in the directional microphone device 10 are shown. As shown in FIG. 5, in the directional microphone device 10, it is possible to obtain directional characteristic at high frequency while overcoming the problem of the increase in thermal noise (decrease in sensitivity), by making only low frequency non-directional.    Patent Reference 1: Japanese Patent No. 2770594